This article proposes an automatic group formation method applying the particle swarm optimization (PSO) algorithm to boost the quality of students' online interactions. The groups were heterogeneous regarding their levels of knowledge and their interests, and three different leadership roles were distributed among group members. A case study with 66 undergraduate students was performed. Discourse analysis was applied using two coding schemes to measure the critical thinking apparent in the students' online discussions and evaluate the socio-cognitive aspects of group interactions. The results provided evidence that groups of undergraduate students formed by the proposed method achieved better scores in most categories analyzed when compared to the randomly formed groups.